Biological activity of bacterial cultures has always been associated with gas production or gas utilization. Cellular metabolism, i.e., biological activity, in aerobic cultures is related to the utilization of oxygen and in anaerobic cultures to the production of methane. In water pollution, the food for the culture or organisms is waste matter and the quantity of oxygen utilized or methane produced indicates the amount of waste matter (food). The amount of food metabolized (or biological activity) is reflected in the rate of respiration or methane production. These rates are particularly useful to control wastewater treatment operations.
The most common method of measuring biological activity of wastewater is the 5-day B.O.D. (Biochemical Oxygen Demand). The 5-day B.O.D. is useful in describing the characteristics of plant influent or effluent and the oxygen demand on the stream, but it is useless for wastewater treatment control because of the five-day delay.
Another measurement which is used to indicate biological activity is to measure the change in volatile solids which is related to the amount of oxidizable material in the food. This measurement does not indicate the rate of respiration (i.e., metabolism) of the food.
Additional methods of measuring biological activity include chemical oxygen demand (C.O.D.), total organic carbon (T.O.C.) and other rapid combustion techniques. The results generally have little relationship to the measurement of respiration rates which is a relatively slow oxidation process that takes place under much different environmental conditions.
The actual biological activity of wastewater, i.e., respiration, is difficult to measure. Ideally, the rate should be measured without removing the sample from its environment. In the standard 5-day B.O.D. test, samples are incubated at average stream temperature (68 degrees, F.), seeded and buffered in an effort to simulate stream conditions.
In an attempt to produce a better indication of biological activity, especially in activated sludge waste treatment plants, dissolved oxygen probes have been placed in aeration tanks to give an indication of the oxygen utilized. At best, dissolved oxygen in a static measure of the concentration of oxygen at time t, but does not indicate the rate of oxygen demand. It the values of D.O. (Dissolved Oxygen) are plotted against time, the resulting graph indicates the rate of change of D.O., not oxygen demand. In effect, the graph is a measurement of the difference between the rate of demand and supply. If the rates are equal, the D.O. is constant, but the oxygen demand could be high. Oxygen demand rates can be used to predict changes in D.O. levels and are therefore particularly useful in control.
The apparatus and method of the instant invention can be used wherever it is necessary to monitor wastewater for information or control purposes. The rates of demand or production can be fed directly into recorders, data processors or controllers. Applications include: